The different cells found in blood may be immunologically distinguished from one another by antigens found on the cell surface. For example, subsets of lymphocytes, the cells of the immune system that specifically recognize and respond to foreign antigens, can be distinguished by “Cluster of Differentiation” antigens or CD antigens, which are cell-surface antigens that can be recognized by specific antibodies. Helper T-lymphocytes that display the CD4 antigen are known as CD4+ T-lymphocytes. CD4+ T-lymphocytes are important to maintaining an immune defense, since they help to induce B-lymphocyte differentiation and production of specific antibodies and to regulate other lymphocyte responses.
The causative agent of Acquired Immune Deficiency Syndrome (AIDS) in humans, Human Immunodeficiency Virus (HIV), primarily infects CD4+ T-lymphocytes. HIV infection and the progress of AIDS disease are marked by a decrease in CD4+ T-lymphocytes. Absolute CD4+ T-lymphocyte counts and the change in these counts over time are clinically important markers of viral infection, disease progression, and therapy efficacy, as well as valuable prognostic indicators in HIV-infected patients. The ability to monitor CD4+ T-lymphocyte levels is important in surveillance of AIDS incidence, in studying the effects of antiviral therapy, and in making decisions about therapeutic strategies, including decisions to initiate prophylaxis for opportunistic infections such as Pneumocystis carinii pneumonia.
The current benchmark method for measuring absolute CD4+ T-lymphocyte counts is based on immunotyping by flow cytometry, usually in combination with a hematology analyzer (“multi-platform” or “dual-platform” technology) (Anonymous, 1997, Morbidity and Mortality Weekly Report, 46(RR-2):1-29). The absolute CD4+ T-lymphocyte count is commonly based on three measurements: a white blood cell count, the percentage of WBCs that are lymphocytes (“differential”) (both obtained by a hematology analyzer), and the percentage of lymphocytes that are CD4+ T-lymphocytes (obtained by flow cytometric immunotyping). More recent “single-platform” technology uses flow cytometry and internal calibration standards to give absolute CD4+ T-lymphocyte counts (Mandy et al., 2003, Morbidity and Mortality Weekly Report, 52(RR02): 1-13). Unfortunately, flow cytometry requires expensive instrumentation and specialized technical training, which generally limits this method to those countries that have the resources to afford it. Both developed countries and underdeveloped, resource-poor countries have a great need for diagnosing and monitoring HIV infection and AIDS disease progression, but the latter often cannot afford routine flow cytometry. The incidence of HIV infection and AIDS disease is high in many such underdeveloped countries, and there is an urgent need for an inexpensive, simple, and accurate method for reliably measuring absolute CD4+ T-lymphocyte counts.